Wrap Directory.watch on Mac OS for the watcher package.

pkg/watcher/test/ready/mac_os_test.dart

Index: pkg/watcher/test/ready/mac_os_test.dart
diff --git a/pkg/watcher/test/ready/polling_test.dart b/pkg/watcher/test/ready/mac_os_test.dart
similarity index 77%
copy from pkg/watcher/test/ready/polling_test.dart
copy to pkg/watcher/test/ready/mac_os_test.dart
index fa4f0cbe268d8b41c43c93ea36c87fb8acf76cad..e0275c4ee2262bda6c6eb514c6a84d3694d7bf34 100644
--- a/pkg/watcher/test/ready/polling_test.dart
+++ b/pkg/watcher/test/ready/mac_os_test.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:scheduled_test/scheduled_test.dart';
+import 'package:watcher/src/directory_watcher/mac_os.dart';
 import 'package:watcher/watcher.dart';
 
 import 'shared.dart';
@@ -11,7 +12,7 @@ import '../utils.dart';
 main() {
   initConfig();
 
-  watcherFactory = (dir) => new PollingDirectoryWatcher(dir);
+  watcherFactory = (dir) => new MacOSDirectoryWatcher(dir);
 
   setUp(createSandbox);